The invention relates to the air conditioning of the passenger compartment of motor vehicles.
For this function, recourse is usually made to a system having a closed loop of refrigerant fluid, the system comprising a compressor, a condenser, a pressure-reduction valve and an evaporator, the latter being in contact with an airflow to be cooled.
Such a system is designed to operate correctly with a minimum quantity of refrigerant fluid. Below this minimum quantity, the evaporator is no longer supplied with sufficient fluid in the liquid state, and exhibits a substantially overheated area that does not allow adequate cooling of the airflow. A lack of refrigerant fluid can also lead to the compressor being damaged. This can come about especially in the event of leakage in the system.
The object of the invention is to allow detection of an insufficient refrigerant-fluid level, without having recourse to sensors provided specially for that purpose.
The invention envisages, in particular, a method for checking the refrigerant-fluid level of a closed loop air conditioning system for air conditioning the passenger compartment of a motor vehicle, the system including an evaporator suitable for cooling an airflow to be sent into the passenger compartment.
According to the invention, a signal value, representative of the cooling power produced by the evaporator, is derived from at least one temperature sensor placed in thermal contact with the evaporator or downstream from it in the said airflow, and the value of the signal is compared with a reference value corresponding to a normal refrigerant-fluid level, a value below the reference value indicating a low refrigerant-fluid level.
To derive the control signal, one or more existing sensors can be used, especially sensors serving to control the air conditioning as a function of cooling requirements or for preventing icing of the evaporator.
The method according to the invention can be implemented either during maintenance operations on the vehicle or while it is in use, a low level of refrigerant fluid then being signaled to the driver, for example by a lamp placed on the dashboard.
Optional characteristics of the invention, which are complementary or alternative, are set out below:
to derive the signal value, at least one temperature sensor is used, placed in a mixing region between the evaporator and at least one air-outlet aperture into the passenger compartment;
to derive the signal value, at least one temperature sensor is used, placed in the vicinity of an airflow-outlet aperture into the passenger compartment;
to derive the said signal value, at least one temperature sensor is used, placed in a region of the passenger compartment swept by the said airflow;
the temperature of the said airflow is measured upstream of the evaporator and the result is processed by means of a model of the operation of the air-conditioning apparatus in order to determine the said reference value;
the temperature of the evaporator, or that of the said airflow in contact with the evaporator, is measured in the vicinity of the inlet for the refrigerant fluid into it, in order to determine the said reference value;
when the air-conditioning system includes a compressor of the variable-throughput type controlled by an electrical control signal, the reference value is calculated from the control signal;
the compressor is of the external-control, variable-displacement type, the control signal being a control current supplying a valve incorporated into the compressor and able to establish, at the inlet to it, a pressure which is determined by the said control current;
the compressor is driven electrically;
the representative signal value is derived when the compressor is operating at a sufficient throughput, in the case of a normal refrigerant-fluid level, to provide correct filling of the evaporator with fluid is in the liquid state;
when the compressor is operating, the airflow leaving the evaporator is reheated so as to be sent into the passenger compartment at the desired temperature;
the reference value corresponds to a temperature a few xc2x0 C., especially about 7xc2x0 C., below a temperature varying as a linear function of the control signal, corresponding to the average value of the representative signal value observed in the normal refrigerant-fluid level case.